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Saturday, March 7, 2009

Science Saturday Challenge #6

Tinkering With Inertia

Newton's first law of motion describes inertia: a body a rest will stay at rest, and a body in motion will stay in motion in a straight line at a constant speed unless either is acted upon by an outside force. We read about inertia in the Dynamics section of the Usborne Science Encyclopedia (pg. 122.) This is not just a linear property, but a rotational one as well.

We used Tinker Toys to explore rotational inertia. (You could also do this with clay and dowels if you want.)

Ds#3 is holding two Tinker Toy configurations that use the exact same pieces. I had the boys hold each one in the center and twist it back and forth, making the top and bottom swing back and forth like a pendulum. I asked which would be easier to swing; they thought the one with the weights at the ends would be.

They were surprised!

The farther the weight, the greater the inertia, so the harder it is to move.

Ds#2 is showing our next experiment. Here I asked the boys to try and balance this structure on their palm. They tried it with the weight closer to their palms...

...and with the weight farther away from their palm. They predicted the first position would be easier to balance.

They were surprised again!

As in the first experiment, the farther the weight, the greater the inertia, so the harder it is to move.

Next Ds#1 is demonstrating another structure that I had them balance on one finger. You see that his finger is at the center of the structure with equal weights in each end.

I asked what they thought would happen if I moved the weight in on one side. Ds#2 thought the side with the closer weight would be heavier, and Ds#1 thought the other side would be heavier. The toy fell towards the weight that was farther away. In the picture, Ds#1 shows that he had to move his finger so that it was midway between the weights, not the middle of the bar, to balance the structure.

Here gravity is acting on the mass creating a force; rotational force is called torque, which is a force exerted at a distance from the axis of rotation. The longer the arm, the greater the torque.

I then showed them how this all relates to something common: a seesaw. I asked which end of the seesaw they would rather have to lift. At first they thought the side with the closer weight, but after a moment they changed their mind.

This time they were right!

Just like the second experiment, a fulcrum is the axis of rotation, so the longer the arm the greater the gravitational torque.

Even without knowing actual values, you can see that the farther the weight is (the greater r is) then the greater the torque is. If you have to lift a weight using a fulcrum, like a seesaw or a pulley, you certainly don't want the short end of the stick!

Another example of this is when skaters spin. With their arms out, the center of mass is farther from the center and the spin is slower than when they bring their arms close to their bodies. This is also why two balls of the same mass but different size will roll down an inclined plane at different rates. The smaller ball rolls faster since less energy has to be spent to overcome the rotational inertia.

When I did this for my co-op class, one child related that it was easier for him to balance his baseball bat with the fat side up rather than down, and one of the moms, who has put together a homeschool color-guard, said that the flags are easier to balance with the flag end up!

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On Science, Learning, and Faith

"Books dealing with science as with history, say, should be of a literary character, and we should probably be more scientific as a people if we scrapped all the text-books which swell publishers' lists and nearly all the chalk expended so freely on our blackboards."Charlotte MasonA Philosophy of Education

"Some teachers may give a live lesson from a stuffed specimen, and other teachers may stuff their pupils with facts about a live specimen; of the two, the former is preferable."Anna Botsford ComstockHandbook of Nature Study

“Science can purify religion from error and superstition. Religion can purify science from idolatry and false absolutes.”

Pope John Paul II

"Beautiful is what we see.More beautiful is what we understand.Most beautiful is what we do not comprehend."Bl. Nicolas StenoFounder of modern geology

"Go my Sons, burn your books and buy stout shoes, climb the mountains, search the valleys, the deserts, the sea shores, and the deep recesses of the earth...Observe and experiment without ceasing, for in this way and no other will you arrive at a knowledge of the true nature of things." Petrus Severinus16th century Danish alchemist

"All this propaganda for literacy of one sort or another comes from people who believe that everyone should share their particular views of what the most important knowledge is and what conclusions should be drawn from it; in other words, they want others to be indoctrinated."Henry H. BauerScientific Literacy and the Myth of the Scientific Method

"Furthermore, and contrary to popular belief, the Church never supported the idea that the earth is flat, never banned human dissection, never banned the zero, and certainly never burnt anyone at the stake for scientific ideas."James HannamThe Genesis of Science